Method and apparatus for control of boundary between electrolytic fluids



I L. G. LONGSWORTH 2,563,729 METHOD AND APPARATUS FOR CONTROL OF BOUNDARY BETWEEN ELECTROLYTIC FLUIDS Aug. 7, 1951 2 Sheets-Sheet 1 Filed Feb. 5, 1945 METHOD AND L. G. LONGSWORTH 2,563,729 APPARATUS FOR CONTROL OF BOUNDARY BETWEEN ELECTROLYTIC FLUIDS Filed Feb. 5, 1945 2 Sheets-Sheet 2 Patented Aug. 7,' 1951 METHOD AND APPARATUS FOR CONTROL OF BOUNDARY BETWEEN ELECTROLYTIC FLUIDS Lewis G. Longsworth, New York, N. Y., assignor to the United States of America as represented by the United States Atomic Energy Commission Application February 3, 1945, Serial No. 576,122

My invention relates to apparatus for maintaining a boundary between fluids of different conductivity and, more particularly, to a device in which one fluid is superposed on another and the circumstances are such that the location of the boundary between them may be maintained or shifted by the addition of quantities of one of the fluids.

For examples, in systems for the electrolytic separation of isotopes of a metal it has been found that the addition of an acid at the cathode will cause a counterfiow which may be controlled to sweep back ions of a heavy isotope and permit the collection at the cathode of the more mobile ions of a lighter isotope. It is known that in such a system therate of eounterflow must be carefully regulated, since an insufficient eounterflow will permit the less mobile ions to reach the cathode while excessive eounterflow will sweep back ions of both heavy and light isotopes. In other words the rate of counterflow must be such that the ions of the heavy isotope will be swept back and the more mobile ions permitted to pass to the region of the cathode.

I have discovered that the rate of eounterflow may successfully be controlled by measuring the conductivity of the electrolyte at a given point in the system, preferably at a location nearer the cathode than the anode, and varying the rate of acid addition proportionally with the conductivity of the electrolyte at the chosen spot. This, in effect, establishes and maintains a boundary between the electrolyte solution and the highly acid solution disposed at the region of the oathode.

One object of my invention is to provide means for automatically regulating the position of the boundary between fluids of differing conductivity, in a system'wherein one fluid is superposed on another. Another object of my invention is to control the rate of eounterflow in an electrolytic system for the separation of isotopes so that less mobile ions are effectually retarded and more mobile ions are permitted to reach the cathode region. Parenthetically, it should be observed that complete separation of the isotopes is not obtained, the end result being a solution enriched as to the concentration of the lighter isotope or isotopes.

Another object of the invention is to provide an automatic, self-regulating control for the eounterflow in an electrolytic system for the separation of isotopes.

A further object of the invention is to simplify and render commercially practical isotope 7 Claims. (Cl. 204-1) separation in an electrolytic system wherein a eounterflow is employed to sweep back less mobile ions and permit the passage of more mobile ions.

An important feature of my invention resides in the combination of a pair of control electrodes inserted in an electrolytic system in a plane normal to the direction of the electrolyzing current, a bridge circuit so connected that the fluid between the control electrodes forms a part thereof, and means responsive to the conductivity of the fluid at these electrodes for controlling the addition of a fiuid to the electrolytic system.

Another feature of the invention resides in a novel electrical control wherein high frequency alternating current is passed through a bridge circuit in which one leg comprises a portion of an electrolyte and in which a thyratron relay is arranged to operate a magnetic burette when the conductivity of the electrolyte varies by a predetermind amount.

Among the advantages which result from the practice of my invention is the fact that the automatic control of the eounterflow makes it possible for a single attendant to supervise the operation of a large number of separators. The expense of operation is still further reduced in View of the fact that the automatic control feature renders the system more efficient than separators in which the counterflow is regulated by an attendant who can only estimate the extent to which additional fluid must be added to the system from time to time.

These and other objects and features of the invention will more readily be understood and appreciated from the following detailed description of one embodiment thereof selected for purposes of illustration and shown in the accompanying drawings in which:

Fig. '1 is a diagrammatic view, partly in crosssection, of an electrolytic system for the separation of isotopes in which my invention has particular application, and

Fig. 2 is a schematic circuit diagram of apparatus employed in practicing the invention.

As shown in Figure 1, an apparatus for the separation of isotopes of a substance may include a U-shaped glass tube I0, provided at one end with an enlarged cup-shaped portion l I into which an anode I2 is inserted. A spillway I3 is formed in the wall of the tube at the anode region H. Similarly, the other end of the U-shaped tube I0 is enlarged to provide a cup-shaped cathode region [4 into which a cathode I1 is inserted. Disposed in the tube H1 in the leg of the U beneath the cathode I! is a plug l6 of sintered glass. A glass cup 18 surrounds the cathode IT and has for its bottom wall a sintered glass diaphragm I9. The anode I2 and the cathode I! are the working electrodes.

It will be understood that the apparatus may be used for the separation of isotopes of any metal of which the ionized isotopes possess different mobilities. By Way of example, potassium has isotopes K and K the ion of K being more mobile in an electrolyte than the ion of K In carrying out the separation, a solution of KCl may be poured into the glass tube it) at the anode end, while HCl solution is periodically added at the cathode as will be more fully explained below. When a potential is applied to the anode and cathode electrodes, i2 and I7, chlorine ions will be drawn to the anode I2 and potassium ions of both isotopes will move toward the cathode l'i. The function of the sintered glass plug 16 is to minimize remixing by turbulence and con-. vection. The plug permits the flow of ions but also quiets the how through the separating zone. The lighter K ions, of higher mobility, will tend to accumulate around the cathode at a faster rate and the less mobile isotope ions, i. e. those of the heavier K but if no further steps are taken and time is allowed to elapse, both isotopes will eventually gather at the cathode ll. However, by adding liquid at the cathode while maintaining a suflicient number of ions (other than K ions) in such liquid to insure the desired electrolytic flow of current between the workin electrodes, the resulting hydrodynamic flow of liquid toward the anode will oppose the electrolytic migration of potassium ions toward the cathode, and if the rate of flow of the added electrolyte is appropriately balanced against the K ion migration, the K ions can be, in eiiect, selectively retarded in an inverse relation to their mobilities.

Thus in the system shown, by the addition of HCl (i. e. an aqueous solution of hydrochloric acid) at the cathode region at a carefully determined rate, the heavy K ions will tend to be swept back more than the lighter and more mobile K ions, or rather, the latter will be more able to overcome the counter flow and will progress further toward the cathode Under presently preferred conditions, the flow is adjusted so that few, if any, of the potassium ions actually reach the cathode, and a socalled boundary between HCl and KCl solutions is thus formed (provided the HCl is added to the solution at the proper rate), advantageously at a point in the system between the cathode and the plug l6, e. g. as indicated by the reference character 20. Under proper conditions the boundary can actually be seen, because of difference of index of refraction of the two solutions. The K 9 ions tend to migrate to the region of this boundary, while the counter-.

flow retards the K ions to a greater extent, so that a gradient of the relative concentration of K to K ions can be established between the boundary and the anode region. But for conand just below the boundary, and by appropriate means, enriched solution can be Withdrawn from this zone. The remainder of the apparatus to be described functions to maintain the boundary between the solution of KCl and HCl.

A pair of platinum electrodes 2 i, i. e. the control electrodes, having their connecting wires encased in glass tubes 22, are inserted from the oathode end of the tube i i and disposed in the plane of. the desired boundary 2s. A burette 2G is dis posed with its lower open end within the glass cup H3 at the region of the cathode ii. A plug 2'1 of sintered glass is disposed in the burette adjacent its lower end, and an inlet tube 28 may communicate with a larger reservoir or container of HCl (not shown). In the lower part of an upper, enlarged portion of the burette 26, and below the inlet 28, is a valve comprising a rubber plu 29 driven into the burette and providing a valve seat with which a closure member 30 is arranged to cooperate.

The closure 36 is carried on the end of an arm 3i mounted for pivotal movement on a pivot 32 and is normally held in contact with the valve seat by means of a tension spring 33. The other end of the arm 31 carries an armature 3,6 coop? crating with a solenoid 3,4 which is adapted to be operated in accordance with the tendency of ie boundary 25 to shift, as will hereafter be explained.-

Ihe circuit shown in Fig. 2 comprises a multivibrator 48 including a twin triode tube 50 of a type well known in the art such as 6N7. lhe high frequency multivibrator itself, which may be in the form of a so-called push-pull negative resistance oscillator, forms no part of my invention and, as shown is composed of elements connected in a manner well known in the art. The electrodes 2| are connected to terminals 4| (Fig. 2), and it will be seen that the electrodes 2| and the fluid between them form one leg of 3, Wheatstone bridge circuit. The output of the multivibrator .40 is applied across the bridge circuit through two primary windings 5| and 52 and a secondary coil 53 (included as the input of the bridge circuit). The output of the bridge circuit is transformer coupled to a triode amplifier tube t2, the circuits of which are arranged in a manner well known to the art. A (-3F5 tube may be conveniently used in the amplifier. A. C. power is applied to the circuit at the terminals 53. The output of the amplifier 42 is applied to a conventional thyratron relay circuit A l. Again, the details of the thyratron relay form no part of my invention, and the relay circuit is constructed and arranged in a manner wellknown in the art. An FG 57 tube may conveniently be used in the thyratron circuit. The solenoid 34 is included in the plate circuit of the relayed, the connections being made to the terminals 66. It is evident that the circuit may be adjusted so thatthe Wheatstone bridge is balanced to deliver a voltage insufficient to fire the thyratron relay its, this condition being obtained when the electrodes 2| are disposed in a relatively concentrated solution of HCl.

When the electrolytic process takes place in the system, the ions of the isotopes of potassium will drift toward the cathode I! and the conductivity of the solution lying between the electrodes 2| will decrease; that is, theboundary will move up, toward the cathode |7. When this happens, the bridge circuit is thrown out of balance, and a pulse will be delivered to the amplifier 62 where it will be amplified and delivered to the relay i4 as a voltage suflicient to fire the 'thyratr'on. The operation of thethyratron will energizethe windings of the solenoid 34, draw down the armature 36 on the arm 3i, and open the valve 30. This permits HCl to flow from the burette into the cup l8, the level of HCl in the cup being indicated by the reference character 31. Hill will continue to be addedto the system until theboundary between theI-ICl solution and the K01 solution is shifted backto the plane 20. The conductivity of the solution between the electrodes 2| will then be restored to its initial value, the "bridge circuit will then be balanced, and the solenoid 34 will be de-energized thus permitting the valve 30 to close and Shut on the flow of HCl through the burette. A condenser 45, adapted to have a filtering or smoothing effect, is shunted across the terminals 46 in the plate circuit of the thyratron in accordance with principles well known in the art, and improves the operation of the solenoid 34. I have found that '20 mfd. is approximately th proper value for the condenser 45.

Although, theapparatus of "my invention functions in a step-by-step manner, the result is a flow of HCl from the cathode region at a rate sufiicient to keep the boundary essentiall stationary and thus-to retardpreferentially the-less mobile K ions while permitting the passage of the more mobile K ions into the region of the boundary. Means may be provided for withdrawing from the boundary region solution which is rich in K as compared to K, for example, one or more tubes 55, conveniently of capillary or near-capillary internal diameter, mounted to ex tend into the liquid below the boundary 20 and provided with stopcocks 56. Whether the withdrawal is continuous or intermittent through means of the sort shown, or whether the operation is simply stopped from time to time to permit introduction of other devices for removing enriched solution, care should be taken to avoid agitation or mixing with unwanted portions of liquid. KOH may be added at the anode from time to time to replenish the supply of potassium ions.

Although lower frequencies may be used in some cases, it seems preferable that the oscillator 40 supply a relatively high frequency to the bridge circuit of Fig. 2, say of the order of 1000 cycles per second or even considerably higher, to facilitate prompt and efiicient response of the thyratron 44 and the solenoid 34 in the circuit shown, and to minimize electrolytic disturbance between the electrodes 2|. Purely by way of example, and with the understanding that other values may be used, or other circuit arrangements, the following are particulars of one satisfactory circuit of the type of Fig. 2 that was used with the tubes mentioned above: Resistors 6| and 62 were 10,000 ohms apiece, condensers G3 and 84 each 0.1 mfd. and condenser 55 was 0.02 mfd. The independent inductance of each of coils 5i and 52 was 30 millihenries, and that of coil 53 was 60 millihenries, the latter being coupled to the former. The tube 42 was operated with a plate supply of 90 volts and a negative grid bias of 3 volts, and the negative grid bias of the thyratron stage 44 was 6 volts, supplied through a limiting resistor 66 of 0.1 megohm. Terminals 43 were connected to 110 volts A. C., and the oscillator plate battery 61 was 45 volts.

Since it is preferable to avoid having the con.- trol electrodes pick up an IR drop due to the electrolyzing current between the working electrodes, the control electrodes 2| in the apparatus shown lie iii a plane normal to the direction of the electrolyzing current, i. e. are disposed :at least approximately on an equipotential surface in the fluid. It will also now be appreciated by those familiar with conductivity measurements, that the electric current passed through the fluid between the control electrodes 2| (as distin'- guished from the electrolyzing current between the working electrodes [2 and H) is preferably kept to a minimum consistent with desired sensitivity and reliability of response.

It will be understood that the apparatus 'of my invention may be used to effect the separation of isotopes of any metal wherein the isotopes are sufficiently different in mass to permit their segregation by a counterflow as disclosed herein. Furthermore, it will be obvious to those skilled in the art that other electrolytes may be used *in the separation of potassium isotopes. For example, KNOs and 'HNOs may be used instead of KCl and HCl. With different metals and different solutions, the rate of counterflow will differ.

In its broad aspects, my invention contemplates controlling the boundary between juxtaposed solutions in an electrolytic system by measuring the conductivity of the fluid at a point in the system and using that measurement to control the rate at which one of the solutions is added to the system. Accordingly, it is to be understood that other types of circuits could be employed for the same purpose and that the particular circuit shown in detail herein is but one example of many different ways in which the invention may be applied.

I claim:

1. The method of establishing and maintaining an ionic boundaryin a liquid medium containing dissolved ions while passing an electric current between electrodes immersed in the medium comprising the steps of establishing a counterflow of liquid to retard migration of said ions toward one of said electrodes inversely in accordance with the mobilities of said ions by supplying an electrolyte substantially free of such ions in the path of the ions toward said electrode, and thereby establishing a boundary between said electrolyte and liquid containing the aforesaid ions that tends to move toward said electrode, detecting change in electrical characteristics of said liquid at a point where it is desired to maintain said boundary, said change being representative of movement of said boundary, and converting the detected change in electrical characteristics of liquid into modification in the aforesaid supply of electrolyte in a direction to counteract the last mentioned movement of the boundary.

2. A method of controlling distribution of ions in a liquid stream which comprises subjecting said ions to the electrolyzing action of charged electrodes in said stream, measuring the conductivity of said stream at a point between said electrodes and controlling the rate of flow of said stream in relation to the conductivity of said stream at said point.

3. A method of positioning an ionic boundary in a liquid stream which comprises subjecting said ions to the electrolyzing actions of charged electrodes in said stream, measuring the electrical conductivity at the desired boundary posi-- tion and controlling the rate of flow of said stream countercurrent to the motion of said boundary toward one of said charged electrodes in response to changes in conductivity of said stream at the desired boundary position.

4. A method of positioning an ionic boundary i in a liquid stream which comprises flowing said stream between two charged electrodes to cause ions in said stream to tend to move in a direction countercurrent to said direction of flow, measuring the electrical conductivity in said stream at a point at which it is desired to maintain said boundary, flowing a liquid which is free from said charged ions past one of said electrodes so as to sweep back the ions attracted toward this electrode and controlling the rate of flow of said liquid responsive to changes in conductivity at the desired boundary point caused by the shift of said ions toward and away from said charged electrode.

5. In electrolytic apparatus comprising two electrolyzing electrodes positioned in a vessel and each immersed in a solution of difierent electrical conductivity, means for maintaining the ionic boundary between said solutions which comprises conductivity detecting electrodes positioned at the desired boundary transversely to the direction of current flow between said electrolyzing electrodes, a valve at one of said electrolyzing electrodes for admitting one of said solutions to said vessel,ra supply container with which said valve communicates, a source of electric potential operably connected across said conductivity detecting electrodes and an electric circuit acting in response to changes in resistance between the said conductivity detecting electrodes to operate said valve and thereby to control the flow of solution from said supply container.

6. In electrolytic apparatus comprising two electrolyzing electrodes positioned in a vessel with each immersed in a solution of difierent electrical conductivity, means for maintaining the ionic boundary between said solutions comprising a pair of conductivity detecting electrodes disposed at the desired boundary normal to the direction of current flow between said electrolyzing electrodes, a solenoid-operated burette at one of said electrolyzing electrodes for containing and ad- .mitting one of said solutions to said vessel, a

source of electric potential operably connected across said conductivity detecting electrodes and an electric circuit acting in response to changes in resistance between the said conductivity detecting electrodes to operate said burette and thereby to control the flow of said solution from the burette.

7. In electrolytic apparatus comprising a generally U-shaped vessel having an electrolyzing electrode in each arm of said vessel with each electrode immersed in a solution of different electrical conductivity, means 'for maintaining the ionic boundary between said solutions which comprises conductivity detecting electrodes positioned in one arm of said vessel relatively close to the electrolyzing electrode therein and normal to the direction of current fiow between said electrolyzing electrodes, a burette at the last said electrolyzing electrode for delivering solution to said vessel, a source of electric potential operably connected to said conductivity detecting electrodes and an electric circuit acting responsively to changes in resistance between said conductivity detecting electrodes to operate said burette and thereby to control the flow of solution from said supply container.

LEWIS G. LONGSWORTH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,145,509 Pike et a1. July 6, 1915 1,388,613 Simsohn Aug. 23, 1921 2,004,569 Davis June 11, 1935 FOREIGN PATENTS Number Country Date 88,280 Switzerland July 1, 1921 302,490 Great Britain Dec. 20, 1928 OTHER REFERENCES Proceedings of the National Academy of Sciences, volume 9, (1923), pages 75 through 78.

Creighton and Koehler, Electrochemistry, 4th edition, volume 1, published 1944 by John Wiley & Sons, Inc.,London, pages 129, 139 and 140. 

